This application claims the priority of Japanese Patent Application No. 2000-093867 filed on Mar. 30, 2000, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a collimator lens used in an optical scanning apparatus, such as copier or laser printer, which scans a laser beam so as to record or display an image; and, in particular, to a collimator lens for converting a divergent luminous flux emitted from a light source such as semiconductor laser into a parallel luminous flux, and an optical scanning apparatus using the same.
2. Description of the Prior Art
Various optical scanning apparatus, such as copier and laser printer, which scan laser beams so as to record or display images have conventionally been known.
Such an optical scanning apparatus is configured such that a laser beam emitted from a semiconductor laser is converted into a parallel luminous flux by a collimator lens, and is deflected as a rotary polygon mirror rotates, and thus deflected beam is focused onto an imaging plane by an fxcex8 lens.
As collimator lenses used in general, many of those constituted by two or three lenses have been known as disclosed in Japanese Unexamined Patent Publication Nos. 61-173214 and 61-147225, since they are mainly required to satisfy axial performances.
Also, as a collimator lens which keeps the amount of curvature of field lower for multi-beam scanning, one constituted by five elements disclosed in Japanese Unexamined Patent Publication No. 11-271606 has been known.
However, the off-axis performances of the collimator lenses constituted by two or three lenses disclosed in the above-mentioned Japanese Unexamined Patent Publication Nos. 61-173214 and 61-147225 are only suitable in a very narrow range where the sine condition is satisfied. In particular, the collimator lenses disclosed in the above-mentioned publications are hard to apply to a case employing a multi-beam system aimed at accelerating the scanning speed or recording a plurality of different information items at the same time upon a single scanning operation, and the like, since the curvature of field is required to be less than 20 microns within the range where the half angle of view xcfx89 is about 2 degrees.
On the other hand, the collimator lens disclosed in Japanese Unexamined Patent Publication No. 11-271606 is disposed near the light source (laser diode or the like), so that its temperature is likely to rise due to the heat from the light source, whereby wavefront aberration may increase, for example.
In view of the circumstances mentioned above, it is an object of the present invention to provide a collimator lens used in an optical scanning apparatus which, when carrying out multi-beam scanning, can suppress various kinds of aberration, such as the amount of curvature of field in particular, to a very small level with respect to respective light beams from a plurality of light sources, and can secure a back focus sufficient for reducing the influence of the heat from the light sources.
For achieving the above-mentioned object, the collimator lens in accordance with the present invention comprises, successively from a parallel luminous flux side, a first lens having a positive refracting power, second and third lenses having a negative composite refracting power, a fourth lens having a positive refracting power, and a fifth lens having a positive refracting power, the second and third lenses being cemented together, one of the second and third lenses having a negative refracting power, the other having a positive refracting power; the collimator lens satisfying the following conditional expression (1):
Bƒ/ƒ greater than 0.6 xe2x80x83xe2x80x83(1) 
where
Bf is the back focus of the whole lens system; and
f is the focal length of the whole lens system.
Preferably, the first lens is a positive lens having a convex surface directed onto the parallel luminous flux side, the fifth lens is a positive meniscus lens having a concave surface directed onto the light source side, and the collimator lens satisfies at least one of the following conditional expressions (2) to (4):
xe2x88x920.54 less than ƒ1/ƒ23 less than xe2x88x920.36 xe2x80x83xe2x80x83(2) 
0.9 less than ƒ45/ƒ less than 1.4 xe2x80x83xe2x80x83(3) 
0.9 less than ƒ4/ƒ5 less than 1.4 xe2x80x83xe2x80x83(4) 
where
f is the focal length of the whole lens system;
f1 is the focal length of the first lens;
f4 is the focal length of the fourth lens;
f5 is the focal length of the fifth lens;
f23 is the composite focal length of the second and third lenses; and
f45 is the composite focal length of the fourth and fifth lenses.
The optical scanning apparatus in accordance with the present invention uses the above-mentioned collimator lens.
The optical scanning apparatus in accordance with the present invention may carry out multi-beam scanning.
The optical scanning apparatus in accordance with the present invention may be a laser printer or a copier.